Verify a transpiler cache entry before any field of it is used

[robobun]

Problem

• A damaged .pile entry is used as written. With output_byte_length zeroed (byte 0x26), bun big.ts prints nothing, exits 0, and keeps the entry: a zero length skips the output hash check in Entry::load (src/jsc/RuntimeTranspilerCache.rs).
• The sourcemap hash is never checked, the size guard adds the stored lengths with wrapping arithmetic, and a flipped module_type byte fails every later run with TypeError: Expected CommonJS module to have a function wrapper.
• A FIFO at the entry path blocks open() forever.

Fix

• The header now ends with a wyhash of the header fields (format version 28, after module_info: fixed-width tagged wire format with one string table per executable #40509 and compile: resolve module record names through the bytecode string table #40677 took 26 and 27). Metadata::decode checks the version and this hash before it returns any field.
• Metadata::verify_layout requires the offsets and lengths to add up to the fstat size (checked_add). Entry::load then checks every section hash, empty sections included.
• The entry is opened with O_NONBLOCK on unix and rejected unless it is a regular file. A rejection takes the existing path: unlink, transpile, write a new entry.
• Verified: test/cli/run/transpiler-cache.test.ts, 4 tests fail on the released bun. Also regression tests 30887 and 28159 and the isolation cache test.

Background

• The cache keeps the transpiled output of source files of 4 KiB or more. get reads the entry before the parser runs, put writes it after.
• An entry is a 110 byte header plus output code, sourcemap, and ES module record. The header stores offset, length, and hash per section.
• A version bump invalidates all entries once. Most parser changes do this.
• transpiler cache: bind section hashes to input and verify unconditionally #35101 and runtime transpiler cache: per-uid root, ownership check, mandatory payload hashes #35747 only drop the hash != 0 bypass and do not catch a zeroed length. Both conflict with main.

Notes

Repro on the released 1.4.0:

D=$(mktemp -d); cd $D; export BUN_RUNTIME_TRANSPILER_CACHE_PATH=$D/tc
python3 -c "print('\n'.join('export function f%d(a: number): number { return a + %d }'%(i,i) for i in range(400))); print(\"console.log('OUT', f7(1), f399(2))\")" > big.ts
bun big.ts            # OUT 8 401
python3 -c "import struct,glob; f=glob.glob('tc/*.pile')[0]; b=bytearray(open(f,'rb').read()); struct.pack_into('<Q',b,0x26,0); open(f,'wb').write(b)"
bun big.ts; echo $?   # prints nothing, exit 0, on every later run too

With this change the second run logs get("big.ts") = InvalidHash under BUN_DEBUG_cache=1, prints OUT 8 401, and rewrites the entry.

Checks, in the order the reader applies them: version, header hash, input hash and length, features hash, layout against the fstat size, section hashes. The writer now stores all three section hashes, the esm record hash included, so the reader has no hash != 0 special case left.

Tests. The mutation table hits the header hash (zeroed output length, zeroed sourcemap length, flipped type, flipped encoding, zeroed header hash), the layout check (re-signed u64::MAX length, appended bytes, truncated file), and the sourcemap hash (flipped body byte). Each row expects the module to print its marker and the entry to be rewritten byte for byte. The positive control rewrites the output section and re-signs both hashes. It proves that a consistent entry is still served from disk and that the hash of an empty esm record round-trips. The FIFO test expects the FIFO to be replaced by a regular entry. works with empty files now also checks that the entry (empty output section) is not rewritten on the second run. The existing module record test re-signs the record and the header instead of zeroing the record hash.

On the released bun: module type flipped exits 1 with the TypeError above, output length zeroed prints nothing, and sourcemap length zeroed, output encoding flipped, bytes appended, and sourcemap byte flipped leave the damaged entry in place. header hash zeroed, u64::MAX, and last byte removed pass there and are controls for the new code paths. The u64::MAX addition aborts a debug build of main (overflow check) and wraps in release.

The LATIN1 arm used to hash the buffer before it compared the read length. It now checks the length first, like the other arms. from_file_with_cache_file_path does one fstat, the same count as before (get_end_pos was an fstat).

Cost on a hit: one wyhash over 102 header bytes, plus the sourcemap hash, which the writer already computed but the reader never checked. An entry grows by 8 bytes.

Not in this change: Entry::save retries a short pwritev with the unadvanced iovec array. The layout check now detects the result. The writer side is tracked separately.

Also run: cargo check -p bun_jsc for x86_64-pc-windows-msvc and aarch64-apple-darwin, cargo clippy -p bun_jsc.

---

no test proof · iteration 6 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/cli/run/transpiler-cache.test.ts

[robobun]

Status: reproduced on the released 1.4.0 with the steps in the Notes block of the description (zero the u64 at byte 0x26 of the .pile, the module then runs as an empty file on every run). Fix and tests are in this PR.

Rebased on main four times (now on 69c6138). #40238 replaced OutputCode with an owning bun_core::String, so the load arms no longer need the scope guards this change had kept. #40374 renamed BunString::empty() to BunString::EMPTY. #40509 and #40677 took format versions 26 and 27 for the ModuleInfo wire format, so this change is version 28 now, and the module record corruption test keeps the wire format parsing from main (including the even padding after the offsets) and re-signs the record and header instead of zeroing the record hash. The verification logic is unchanged. All review threads are resolved.

CI on the current head (build 107623): every lane that exercises this change is green, including test/cli/run/transpiler-cache.test.ts on every platform that ran it. The two red jobs are unrelated: the darwin aarch64 test job failed before any test ran (buildkite-agent artifact download timed out after 120s), and the darwin x64 job fails in test/js/web/url/url.test.ts (TypeError: Invalid URL), which CI marks as already failing on main. Both are reported to main-break triage. Ready for a maintainer.

[coderabbitai]

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---

Walkthrough

Changes

The transpiler cache format advances to version 26. It adds fixed metadata headers, header and section hash validation, exact layout checks, regular-file enforcement, and expanded corruption tests.

Transpiler cache integrity

Layer / File(s)	Summary
Metadata and layout contract src/jsc/RuntimeTranspilerCache.rs, src/jsc/error.rs	Metadata uses a fixed-size hashed header. Decoding validates metadata fields and cache layout. New errors identify invalid layouts and non-regular files.
Cache writing and reading src/jsc/RuntimeTranspilerCache.rs	Cache writes hash every section. Cache reads validate exact lengths, file layout, regular-file status, and hashes, including empty sections.
Cache corruption and reuse tests test/cli/run/transpiler-cache.test.ts	Tests cover empty-output reuse, damaged entries, malformed layouts, section corruption, FIFO files, and self-consistent cache edits.

Suggested reviewers: jarred-sumner

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the main change: validating transpiler cache entries before using their metadata or contents.
Description check	✅ Passed	The description explains the problem, implementation, verification steps, affected tests, and known scope. It provides the required information even though it uses Problem, Fix, and Background heading…

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Explanation

The description explains the problem, implementation, verification steps, affected tests, and known scope. It provides the required information even though it uses Problem, Fix, and Background headings instead of the exact template headings.

---

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[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@test/cli/run/transpiler-cache.test.ts`:
- Line 15: Replace the tmpdirSync import with tempDir from harness, and update
the temp_dir setup in the transpiler-cache test to call tempDir while preserving
the existing temporary-directory behavior.
- Around line 69-72: Update fileIdentity to use bigint-based file stats and
return nanosecond-resolution timestamp data instead of mtimeMs; retain inode
information where available so reuse assertions reliably distinguish files
rewritten within the same millisecond, including configurations where inode is
zero.

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[claude]

I reviewed this PR and didn't find any bugs. Since it changes the on-disk cache format (version bump + trailing header hash), refactors the Metadata::encode/decode signatures, and adds a cfg-gated O_NONBLOCK open path, a maintainer sign-off would still be worthwhile.

Checked: header offset arithmetic in the test constants matches Metadata::encode byte-for-byte; Bun.hash.wyhash and RuntimeTranspilerCache::hash are the same wyhash-final4 with the same seed, so signPileHeader round-trips; decode on a short read errors in read_int_le before the bytes[..FIELDS_SIZE] slice is taken; verify_layout uses checked_add and pins each offset to the previous section's end plus the file size, so a re-signed u64::MAX length is rejected; O_NONBLOCK on a regular file is a no-op for pread, and a FIFO opens immediately then fails ISREG and hits the existing unlink-and-retranspile path.

Extended reasoning...

Overview

This PR hardens the runtime transpiler cache reader in src/jsc/RuntimeTranspilerCache.rs against damaged .pile entries. It adds a trailing wyhash of the 102-byte header (bumping the format to version 26 and growing the header to 110 bytes), replaces the wrapping-add size guard in Entry::load with a new Metadata::verify_layout that requires the three sections to sit back-to-back and add up to the fstat size via checked_add, drops every hash != 0 bypass so empty sections are hash-checked too, opens the entry with O_NONBLOCK on unix and rejects non-regular files, and reorders the LATIN1 arm to check the read length before hashing. encode now writes into a fixed [u8; SIZE] and decode returns a fresh Metadata instead of mutating one in place. Two new error variants land in src/jsc/error.rs. The test file gains a pile constants block, three new tests (a nine-row damage matrix, a positive control that re-signs an edited entry, and a FIFO test), tightens works with empty files to assert the entry is not rewritten on the second run, and updates the pre-existing module-record corruption test to re-sign the entry instead of zeroing the hash.

Security risks

None material. The cache lives under a per-user directory (BUN_RUNTIME_TRANSPILER_CACHE_PATH, XDG_CACHE_HOME, or ~/.bun), and the change strictly tightens what the reader accepts — every path that used to trust a header field now checks it against a hash and against the file size first. The header hash is wyhash, not a cryptographic MAC, but that matches the existing threat model (integrity against disk errors and torn writes, not against an attacker who already controls the cache directory); the positive-control test demonstrates that a self-consistent entry is still served. The FIFO/regular-file check removes a hang vector.

Level of scrutiny

Medium-to-high. The transpiler cache runs on every bun run of a source file ≥4 KiB, and this PR bumps the on-disk format version (invalidating every existing entry once) and refactors the encode/decode API. The logic itself is straightforward and defensively written, but a format change plus a hot-path reader refactor is the kind of change a maintainer should sign off on rather than land purely on automated review.

Other factors

The test coverage is thorough and structured the way the review guide asks: a mutation table with a per-row expected result (so a single toEqual shows which row failed and how), a positive control that proves the checks are what rejects a damaged entry, and per-test fileIdentity assertions using bigint mtimeNs/ctimeNs after the CodeRabbit follow-up. The PR description states four of the new rows fail on the released 1.4.0 and names which ones are controls. I verified the test-side byte offsets against Metadata::encode field-by-field, that Bun.hash.wyhash is the same bun_wyhash::Wyhash::hash the cache uses, that a short pread_all into the header buffer makes decode fail in read_int_le before the bytes[..FIELDS_SIZE] slice is indexed, and that sys::S::ISREG(st_mode as _) and usize::try_from(st_size) follow the same pattern used across the tree. All prior bot feedback (comment length, fileIdentity resolution, tmpdirSync scope) is resolved on the timeline.

[robobun]

^{Updated 3:33 AM PT - Aug 28th, 2026}

❌ @robobun, your commit c6a77f6 has 2 failures in Build #107623 (All Failures):

• test/js/web/url/url.test.ts - code 1: TypeError: Invalid URL on 🍎 any x64
• step failed outside runner - exit 1 on 🍎 any aarch64 - test-bun

---

🧪   To try this PR locally:

bunx bun-pr 39717

That installs a local version of the PR into your bun-39717 executable, so you can run:

bun-39717 --bun

[claude]

I reviewed this PR and didn't find any bugs. Because it changes the on-disk cache format (version bump, trailing header hash) and adds O_NONBLOCK to the open path on the runtime transpiler hot path, a human look would still be worthwhile.

What was reviewed:

• Metadata::decode short-read safety — the read_int_le calls fail before the &bytes[..FIELDS_SIZE] slice is reached, so no panic on a truncated header.
• verify_layout — checked_add chain is correct; UTF16 odd-length is rejected; offsets must equal the writer's back-to-back layout and sum to st_size.
• O_NONBLOCK on regular files is a no-op for pread; on a FIFO the open returns immediately, ISREG fails, and the unlink guard removes it before re-transpile.
• Test pile offsets match Metadata::encode byte-for-byte; the positive-control test self-verifies that Bun.hash.wyhash matches the Rust Wyhash::hash(SEED, ...).

Extended reasoning...

Overview

This PR hardens the runtime transpiler cache reader against damaged .pile entries. It touches three files: src/jsc/RuntimeTranspilerCache.rs (~200 line diff: format version 25→26, new trailing 8-byte header hash, Metadata::decode refactored to return-by-value, new verify_layout with checked_add, all section hashes now checked including empty sections, O_NONBLOCK + S::ISREG gate on the open), src/jsc/error.rs (two new error variants), and test/cli/run/transpiler-cache.test.ts (~200 new lines: 9-case damage matrix, positive control, FIFO test, and updates to the existing module-record corruption test to re-sign the header).

Security risks

None identified. The cache stores bun's own transpiled output in a per-user directory; the threat model is disk corruption / torn writes, not an adversary. Wyhash is not cryptographic, but the header hash + layout check + per-section hash together are the right defense for that model. The existing unlink + re-transpile fallback path is reused for every new rejection.

Level of scrutiny

Medium-high. This runs on every ≥4 KiB module load, and it changes an on-disk binary format. The version bump correctly invalidates existing entries so there is no back-compat concern, but the format change, the new open flags, and the fstat-derived layout check are the kind of thing a maintainer should sign off on rather than land purely on automated review.

Other factors

I verified: the sys::S::ISREG(stat.st_mode as _) pattern matches other call sites (e.g. ansi_renderer.rs, DirectoryRoute.rs); sys::O::NONBLOCK is defined for unix and correctly excluded on Windows via cfg; read_int_le errors on short input so decode cannot panic on a slice; pread_box(_, 0, _) returns an empty box and verify_hash(&[], ...) matches because the writer now stores hash(&[]) for empty sections; the syscall count is unchanged (file.stat() replaces the old get_end_pos() fstat). All prior review threads (CodeRabbit fileIdentity precision, comment-cop length nags) are resolved. Test coverage is thorough — the damage matrix covers header hash, layout, section hash, and overflow; the positive control proves consistent entries are still served; the FIFO test has a 30 s SIGKILL guard so a regression fails rather than hangs. No CODEOWNERS on the touched paths.

[claude]

I reviewed this PR and didn't find any bugs. The header-hash + verify_layout design is sound, the checked_add chain and section-hash checks close the holes described, and the test matrix covers each new guard plus a positive control. Because it bumps the on-disk format version and changes the open flags on a path every ≥4 KiB module load takes, a maintainer sign-off would still be worthwhile.

Checked: decode slicing bytes[..FIELDS_SIZE] is safe because read_int_le fails first on a short buffer; O_NONBLOCK is a no-op for pread on regular files; pread_box(_, 0, _) and verify_hash(&[], _) round-trip the empty-section hash the writer now stores; the test's pile offsets match Metadata::encode byte for byte.

Extended reasoning...

Overview

Three files. src/jsc/RuntimeTranspilerCache.rs bumps the cache format to v26, appends a wyhash of the 102 header bytes, rewrites Metadata::decode to return a value instead of mutating (checking version and header hash before any field is trusted), adds Metadata::verify_layout (offsets + lengths must checked_add to the fstat size, UTF-16 length must be even), opens the entry with O_NONBLOCK on unix and rejects non-regular files via fstat, and drops every hash != 0 bypass so empty sections are hash-checked too. src/jsc/error.rs adds InvalidLayout and NotARegularFile variants. test/cli/run/transpiler-cache.test.ts adds a nine-row damage matrix, a positive-control re-signed entry, a FIFO test, and updates the existing module-record test to re-sign instead of zeroing the hash.

Security risks

None introduced. The change is defensive hardening: it fails closed (unlink + re-transpile) on any inconsistency in a file the process itself wrote earlier. No new user-controlled input reaches this code — the cache path is the same as before, and the entry contents are now more strictly validated, not less.

Level of scrutiny

Medium-high. The transpiler cache sits on the hot path of every module load ≥4 KiB, and a format-version bump invalidates every existing entry once. That said, the failure mode is benign (re-transpile), the mechanism is clearly explained, and the test suite proves each new guard fires on the released binary. The refactor of encode/decode to fixed-size arrays and value-returning decode is straightforward.

Other factors

All prior review threads (coderabbit on fileIdentity resolution, comment-cop on doc-comment length) are resolved in later commits. The PR description includes a working repro on 1.4.0 and names which of the nine damage cases fail on the released build vs. which are controls for the new code. I verified sys::S::ISREG(stat.st_mode as _) and file.stat() match established patterns elsewhere in src/, and that FixedBufferStream::read_int_le errors on a short buffer so the &bytes[..FIELDS_SIZE] slice in decode cannot be out of bounds. Deferring only because a format bump on a runtime hot path is the kind of change a maintainer should acknowledge, not because anything looks wrong.

[claude]

Beyond the inline nit, I also checked: &bytes[..FIELDS_SIZE] in Metadata::decode cannot panic on a short file — the preceding read_int_le calls return an error before that slice is reached; sys::S::ISREG(stat.st_mode as _) and the O::RDONLY | O::NONBLOCK open are established cross-platform patterns (FileReader.rs, node_fs.rs, Image.rs); and the as usize casts on section lengths in load are bounded because verify_layout already matched their sum against usize::try_from(st_size).

Extended reasoning...

The inline finding is a test-quality nit only. During review I traced the new decode/verify_layout/load path for panic safety on adversarial input and confirmed the platform-gated open flags and S::ISREG follow existing in-tree usage, so noting those here to save the next reviewer re-deriving them. Not approving: this is a cache-format version bump plus a refactor of the encode/decode signatures and new verification logic on a hot path — a maintainer should sign off on the format change and the O_NONBLOCK approach.

[claude]

I reviewed this PR and found no bugs; the earlier nit about signalCode in the FIFO test was addressed in d07ec22. Because it bumps the on-disk cache format and reworks the hot-path entry reader (header hash, layout verification, O_NONBLOCK open, regular-file gate), a maintainer sign-off on the format/approach is still worthwhile.

What was reviewed:

• decode short-read handling — a file shorter than the header errors in read_int_le before the bytes[..FIELDS_SIZE] slice is reached, so no panic.
• verify_layout — checked_add chains through Option, so an overflow or offset mismatch reads as None != Some(...) and returns InvalidLayout.
• O_NONBLOCK on unix — no-op on regular files per POSIX, so pread_all behaves as before; a FIFO opens immediately, fails ISREG, and hits the existing unlink-and-retranspile path.
• Empty-section round-trip — writer now stores hash(&[]) for every section and the reader checks it, verified by the extended "works with empty files" test.

Extended reasoning...

Overview

The PR hardens the runtime transpiler cache reader against damaged .pile entries. Metadata gains a trailing wyhash of its 102 field bytes (format version 25 → 26). decode becomes a constructor that returns a Metadata only after the version and header hash check out. A new verify_layout requires the three section offsets/lengths to add up (via checked_add) to the fstat size and requires an even UTF-16 output length. Entry::load now verifies every section hash — including the sourcemap (previously never checked) and empty sections (previously bypassed). from_file_with_cache_file_path opens with O_NONBLOCK on unix and rejects non-regular files, so a FIFO at the entry path no longer hangs open(). Two error variants (InvalidLayout, NotARegularFile) are added. The test file adds a damage-matrix test (nine mutations, each expected to be rejected and the entry rewritten byte-for-byte), a positive control that re-signs an edited entry, a FIFO hang-guard test, and updates the existing module-record-corruption test to re-sign instead of zeroing the hash.

Security risks

None in the auth/crypto sense. The change is defensive: it tightens validation of bun's own cache output, and the failure mode is "unlink the entry and re-transpile", which is what happened before for any load error. The header hash is wyhash (not cryptographic), which is appropriate — the threat model is disk corruption / torn writes, not an adversarial writer, and the PR description is explicit about that. The checked_add chain prevents the previous wrapping addition from letting a hostile length past the size guard.

Level of scrutiny

Moderate-to-high. This is not a mechanical change: it bumps an on-disk format version (invalidating every existing entry once), rewrites the header decode from mutate-in-place to construct-and-return, changes the LATIN1 arm's check ordering, and adds platform-gated open flags. The transpiler cache read path runs for every source file ≥ 4 KiB on every bun run, so a regression here would be very visible. That said, the change is strictly additive validation on a path whose failure already falls through cleanly to re-transpilation, the version bump makes the format change safe, and the test coverage is unusually thorough (each check has a targeted mutation, plus a positive control proving a consistent entry is still served).

Other factors

All review threads are resolved: CodeRabbit's fileIdentity resolution concern was addressed with bigint mtimeNs/ctimeNs; the comment-cop length warnings were fixed; my own inline nit about asserting signalCode in the FIFO hang-guard test was addressed in d07ec22. The PR description records that cargo check was run for the Windows and macOS targets. I traced the short-read case through decode (a buffer shorter than 110 bytes errors on a read_int_le before the &bytes[..FIELDS_SIZE] slice, so no OOB), verified the test's byte-offset table against Metadata::encode's field order, and confirmed that Bun.hash.wyhash matching the Rust Wyhash is proven by the positive-control test rather than assumed. Deferring only because a format bump plus a hot-path reader rewrite is the kind of change a maintainer should sign off on; I found nothing to block.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.